Not many interviews begin without their interviewee. Harvey Pekar, however - cult hero of three decades' worth of his own comic book American Splendor - is not here. He's in the bathroom. He has been in the bathroom for some time. This, according to his wife and collaborator Joyce Brabner, should not be an obstacle to interviewing him. "Oh, he'll be out soon," she says. "But we should get started anyway. Really, it's fine. Give me a question. After 20 years I know what he'd say."

Then again, despite being here (or not) to promote a movie devoted to his life, Pekar is hardly your average breathless self-publicist. Neither, although his comics have brought him a certain degree of wide-eyed adulation, is he any kind of superhero. What he is is a famously crabby retired hospital file clerk, a 64-year-old underground icon whose ongoing autobiography in the pages of his (until recently) self-published comic has now been transformed into a movie, also called American Splendor, co-directed by Robert Pulcini and Shari Springer Berman.

And here, in three spruce and engaging acts, is the A-Z of Harvey Pekar: the birth of the comic in grim mid-1970s Cleveland, Ohio, based around its creator's then-revolutionary idea of documenting normal, blue-collar life in all its mundane glory; the slowest of slowburn success stories plus marriage to Brabner (a comics aficionado from Delaware) in the 1980s; then, most recently, a battle with lymphoma that almost killed him and led to his and Brabner's acclaimed record of that struggle, Our Cancer Year. There's even - by way of his recovery - a happy ending.

But it's also a false one: because the movie has brought enough attention to its subject to constitute a postscript, one in which Pekar and Brabner travel the world discussing the original American Splendor. And Brabner is making the most of it, talking fast on her husband's behalf about the overlap of documentary and fiction. Until finally she looks up: "So, there he is."

Enter Pekar, a man whose expression suggests a profound anxiety about something he can't quite put his finger on. "Do you think his hands are clammy?" Brabner asks him (earlier I worried out loud they might be). Pekar seems indifferent. "It's OK, there's worse things to be than clammy." He clears his throat and it still sounds like sandpaper. "So, did you get started already?"

Slumped there, the resemblance between him and his on-screen alter-ego is uncanny. That's partly a tribute to the actor playing him, Paul Giamatti, who delivers less an impression of Pekar than a channelling of his very essence. But even more so, it's testimony to the craft of the movie, an unfailing delight that takes a wildly innovative approach to traditional storytelling and manages to blur all possible lines between Giamatti's Harvey, the comic-book Harvey, and the actual flesh and blood variety seated here.

Pekar was, he says, pleased that the film took such narrative risks; just as quickly, he points out it was scripted that way from the start, so it's not like he can take any credit for it. He looks anxious again. Did he feel it presented an accurate account of the past 30 years of his life? "Well, there were issues of privacy, so some stuff wasn't in there. Mostly just stuff about our kid [Danielle Batone, the couple's adopted daughter]. And there were times they compressed events or what have you... I mean, I don't know what's normal because I don't see too many movies... but yeah, it felt right. It felt true."

He trails off. Brabner picks up the thread: "People have certainly commented to me that in person I seem a lot funnier and more energetic than I do in the film. But that's because the film is really Harvey's way of seeing me, and of course Harvey sees me as this ball and chain who won't get out of bed and refuses to work..."

"But," Pekar says, "it doesn't - " "But it doesn't matter," Brabner says, "because artistically the film is in the right spirit and at least they put across that Harvey is a married guy now. He isn't this lonely bachelor, sitting at home with a bowl of petrified chilli in the fridge."

Indeed he's not, and hasn't been for 20 years. But he certainly was when, back in 1976, he first sat down and - motivated by "sheer frustration" - began scrawling down his thoughts in comic form, screeds of text issuing from the mouths of stick figures that were then transformed into portraits of him by an ever-changing group of artists that included, most famously, close friend Robert Crumb. Pekar's unique way with the minutiae of ordinary life soon began to draw a crowd.

"I just always thought that was what life was actually about," he says. "And yet back then no one talked about it or wrote about it. And there was all this stuff going on in my life and everyone's life around me that was odd and difficult to get a handle on sometimes but also kind of funny and just... real, you know?"

The result, hinted at if not fully explored in the film, was a strange kind of double life. On the one hand, Pekar carried on working in the same hospital he had since the 1960s for a salary a couple of steps up from minimum wage; on the other, he was attracting the kind of cult kudos that saw him being compared to Dostoevsky and hailed as the godfather of a whole new strain of underground literature.

Through it all, however, the one constant was Pekar's omnipresent neuroses, a set of anxieties that would make Woody Allen look like Hugh Hefner. And today, it seems, they're raging. "You know," he finally admits, "I haven't been feeling good these last few days..." He turns to Brabner (they decided to marry, touchingly enough, while she was being sick from a food allergy immediately after their first date). "OK, I'm being honest here, and I haven't really talked to you about this, but I am feeling really anxious about what happens when we go back to Cleveland, and about money, and Danielle and, you know..."

"He's obsessive-compulsive," Brabner shrugs. "And depressive. It's a great combination." Except there's more still to Pekar. There's his status as a critic of both modern jazz and esoteric fiction, a secondary career he says he's desperate to do more of. Politically radical - he speaks of his president with an unfakeable loathing - and a true working-class autodidact, he is not, in the words of his wife, "the kind of American that often gets exported by our pop culture".

And yet export him it has, sending him to a glut of film festivals, sun-addled Cannes included, to explain what he does to people who clearly first heard of him two minutes before. Has that annoyed him? "Nah, doesn't bother me," he says. "It's pretty much routine. I mean, I think it's kind of incumbent on you to do that, and not to make a fuss about it."

For a moment, he almost seems, if not relaxed, then at least resigned. At this stage of the game you could certainly forgive him for kicking back and enjoying the fruits of his labours. The movie could sell a few more comics, he supposes. It might secure him a few writing assignments. But he doesn't sound optimistic. "He doesn't understand optimistic," says Brabner. "We have some money in the bank, we're not going to starve, but he still just catastrophises all the time with this relentless, apocalyptic gloominess. All Harvey can think is 'God, I don't know what I'm going to do, we've got to put this kid through college, the roof needs fixing, there's no money coming in . . . What the hell do I have to be optimistic about?'"

And Pekar's face lights up for the first and only time today. "I'm not dead," he says, eventually. "Well... not yet."